Bilge Problems: Newsletter Challenge (10/10/06)

The seemingly hole is at waterline possibily is from a thru hull fitting. when the weight of leaking water drains to lowest bildge point it changes the position the boat was floating in enough so the area of leak is raised above old water line and water can't flow up hill to get into the boat. DonnieH

Mr. H you are correct! The official answer was bogus!

I agree, the official answer is bogus, as an all encompassing answer, due to the assumptions one has to make. My first answer, (#6) also agreed with Mr. H. However, I re-evaluated that based on re-reading the challenge and came up with an entirely different answer, based on a different assumption (see #32), which I believe is equally valid, for its lack of, or different, assumptions.

No wonder equally competent scientists draw opposite and seemingly contradictory conclusions and proposed theories. Although some just miss the boat (pardon the pun).

Ordinarily, I'd say the $64,000 should go to guest in post 4 who proposed his solution before my first post. However, I, and only I, went to great effort to support his position (and mine), and he seems to have vanished. Therefore, I am claiming the award for myself. If anyone would like part of said award, just send me your bank account numbers, and I'll do a wire transfer.

What $64,000 prize? I would tell the writer of the "official answer" to put their money where their mouth was, but obviously, there is no money.

I still contend there are multiple "right" answers depending on what you must "assume", including the assumption that there actually IS a leak in the boat, and that a "fixed level" means including the level previous to the one observed as fixed, i.e. not increasing or decreasing. I contend that this level was fixed once the water was no longer being removed and reached a new equilibrium between the "compartments", and that it was lower than the previously "fixed" level. I believe the "leak" was an internal one, i.e. the drain holes, not an external, through-hole leak. There is nothing in the challenge that proves otherwise.

My original answer was the same as the official one but I agree, it's pretty bogus. I prefer your answer at #32.

My distaste for the official answer is that I reckon you'd be battling to get the situation in real life. Remember you only sponged out a quart of water and this is the lowest part of the bilge. What sort of boat only has a quart of displacement at the lowest part of the bilge? The scenario described can happen up in the bows of a boat where the sides are steep and water line is close but down at the bottom of the bilge?

The only vessel I can picture that would fit the bill would have to have a double skin hull with much sealed foam between the skins so the "lowest part of the bilge" is not really the bottom of the boat. I'd be interested to know of any other design that could perform in this manner.

I agree that the 1 quart/1Kg of water that gets sponged out seems to make equal water levels pretty unrealistic. If that were the case then the displacement would be something like 1Kg times the number of transverse stiffeners. I suppose you could have a couple of hundred stiffeners but I find that pretty unlikely as well.

I agree with STL, Masu, and Nutwood. I've built several boats, have sailed thousands of miles and have sponged out many bilges. Had I built a boat without limber holes in the floors (transverse stiffeners) I would have been embarrassed. Sponging out the bilge would then involve sponging out many compartments, some of them very difficult to reach.

My Windrocket has about the tiniest possible hull, not much larger than a windsurfer. It's made of very thin material, and therefore has many internal stiffeners (stringers and 8 floors). It draws about 4". Therefore, if I'd failed to provide limber holes, and had a leak in one section, that section would fill to 4" deep (by about 20" wide [geo average] by 24" long: 1920 cu in, or 31 liters (31 Kg). So even in this impossibly tiny boat with more than the usual number of compartments, one compartment would hold 31 times the volume in question.

This reminds me (as have several other challenge questions here) of the story about the physics teacher who asks "How could you use a barometer to measure the height of a tall building?" The student who can parrot the desired answer would talk about measuring the barometric pressure at the top versus bottom, and doing the math. But the bright student would think to himself "Well I can't possibly get even remotely close that way, because the barometer' resolution is so low. Maybe if I dropped it of the top of the building and timed its fall, I could get closer." The story goes on like this, proposing many possible answers based on physical principles, but the best answer is this: "I'd take the barometer down to the basement, find the janitor, and say 'I'll give you this nice barometer if you can tell me the height of this building.' (The janitor, of course, knows the building superintendent, who can find the drawings, and give the measurement very precisely.)

So, do you take the low road and give the answer that they obviously want (which I did, because I really want that prize money -- it's $64,000 right?) or take the high road and propose a more reasonable answer?

Your check valve on the bilge pump is leaking, allowing the water trapped between the pump and the hopefully above the waterline thru-hull fitting to leak past until the check valve can close on the reduced head pressure.

find the guy who designed the boat and kick his ass for building sub standard hulls. Get it fixed by someone who actually knows what the hell they are doing.

go fishing

The bilge of the boat is separated into sections by the transverse stiffeners, kind of like short bulkheads. They run side to side. The leak is in one of these sections only. The top of the stiffeners is higher than the waterline preventing the water from overflowing from one section to another. Once the water in the section is equal in height to the water outside, the water will stop flowing. It has reached equilibrium. Add more stuff to the boat (people, coolers, etc) and the water level will increase. Remove stuff from the boat and the water level will decrease. Think of it as an ice tray. A leak in only one compartment will not sink the tray.

There is a leak in one of the ridges at the same hight as your water puddle... just as a water level keeps the two sides even/plumb so will the leak in the ridge .. sort of like a self watering pet dish.

I would agree with Guest #1, the weight of water in the bilge changes the angle of the water line. A leak near the bow could be below the waterline when the bilge is empty, but as it fills and the water drains into the stern, the bow comes up, bringing the leak above the waterline. The weight of the person checking the bilge in the stern just accentuates the situation, allowing more leak when he moves toward the bow, shutting off the leak when checking and sponging the bilge. If the leak is that small, the boat will also rise and raise the leak above the waterline (or rather the waterline will be lower on the boat) as soon as the person leaves the boat, whether or not the bilge has refilled yet. It could be that he comes back to a dry boat, but it leaks again as soon as he steps on board anywhere between the center of gravity and the bow.

My only concern is, why were the transverse ridges mentioned? Was it just extraneous information, provided to through off the reader, or do they somehow come into play?

Are there multiple answers to this one?

After further consideration, I have deduced the following:

1. The boat is docked and is being left at the dock, therefore it is a big boat, probably 30 feet or more, not conducive to being trailered home.

2. With a fiberglass hull, and the age not mentioned, any leaks will be from cracks, thru-hulls, on-board tanks, rain accumulation, screws thru hull, maybe osmosis. Other than osmosis, and on-board tank leaks, the leak will seek the level of the waterline. By osmosis, I mean water passing thru a blister on the outside hull, thru the porous mat, and accumulating on the interior of the hull, not osmosis in the true sense.

3. If a quart of water in the bilge will affect the water line of your boat, then you need to grab your dinghy, pull it out of the water, and leave it upside down on the dock.

Your scenario is certainly one possibility although there is nothing in the challenge that requires the boat to be large. That is your inference or assumption only. Many smaller boats are left at docks and NOT "trailered home" during the boating season. Whenever my Dad had a ski boat when I was a kid, that was his preference. That way we could just jump in the car and head for the lake, without worrying about trailering the boat or making room for it in our driveway. Depending on boat size, what kind of deal he could cut with the marina owner, and his whim that year, he might just leave it in dry storage at the Marina over the winter or bring it home to sit at the back of the driveway.

We never had a dinghy on any of our boats. Of course, they were all motor boats and easily docked, beached, or anchored in shallow water. The only time we had a problem with the bilge was when my older brother left the stopcock out, which was unnecessary when the boat was running and the bilge drained out automatically, but had to be replaced when docked or the boat would sink. It was a good thing he tied it securely to dock. Only about one third of the boat was underwater when we got there the following week-end. Of course it was the stern and the engine compartment was flooded, requiring dissassembly and cleaning of the motor.

The only cracks that appeared were the ones Mom made about the boat being "a hole in the water which you throw money into."

I believe that what happens is that once the water reaches the level of river it does not have because to continue raising the level. By the same reason the level does not raise if we threw water within the boat, since this escapes by the scuppers.

Say whaaat?????

I'd have to think that the transverse stiffeners are a key to the solution. If they extend above the waterline (as they could in a motorboat) and if they are watertight, then filling one compartment (between stiffeners) to the waterline level won't sink the boat. Once the water in the compartment reaches waterline level it will rise no further.

Most boats have limber holes in such stiffeners to allow bilge water to flow to the lowest point. In this boat the limber holes are either clogged or missing (maybe because the designer envisioned exactly this scenario).

Hey dudes, what about Archimedes (throw some fiber glass on water and it floats), water surface tension, balancing of pressure between 2 volumes, etc....

Since there is a bilge I'll assume the boat is an inboard, although an inboard/outboard engine compartment is referred to as a bilge as well. In this scenario water lmay seep around a worn or weakened glans seal of the prop shaft, continuing until enough water accumulated for its "weight" to reduce the relative pressure against the seal to the point where the seal would once again become effective. Were it not for the openings in the web frames to allow for water taken on board to flow to lowest point aft, water could fill the aft most space between the web frames to the waterline with little effect on how the boat actually rests in the water. As for "the weight" of the water lifting the bow, I can only point out that the water inside the hull below the water line "weighs" nothing per se', and only the materials which weigh more than water can contribute to a boats actually sinking. Although the material weight out of water can contribute to the boat taking on water by forceing the leak under and thereby increasing the water preassure (hence rate) of the leak. As a boat goes down at the stern through loss of buoyancy (taking on water), the bow does not rise until the stern actually goes completely under, and as the dead weight of the heavier than water materials of the sunken end (minus the bouyancy of the materials lighter than water) swings down under the still buoyant air filled bow. Air trapped in the forward spaces will hold the bow out of the water in a direct relationship between its relative buoyancy vs the dead weight hanging below. The reason you see a bit of the bow and foredeck sticking out of the water at a slant when a boat has gone down dockside (they would be bow vertical if unteathered), is that a boat will usually only sink to the depth allowed by the length of the slack in the stern line , while the bow remains slightly bouyant.

Hmmmmm, probably should have used specific gravity rather than the lay "relative weight" reference. Oh well, too late now.

I take exception to two statements you made. They are illogical and since your refutation of my theory is based in large part on these two statements, I cannot accept your argument.

Firstly, you said,"water inside the hull below the water line "weighs" nothing per se', and only the materials which weigh more than water can contribute to a boats actually sinking."

So, If I have a boat that has about 1 ft. of draft (distance from keel to waterline) when it is empty and I fill it with water to within one inch of the original waterline, by your statement, since this water is below the waterline, it weighs nothing "per se'" (whatever that means). Are you telling me that this large amount of water will have no effect on sinking the boat? Hogwash. Putting such a weight of water in the boat will increase the draft, "sinking" the boat deeper in the water, with a new waterline much closer to top edge. If I proceed to pour more water into the boat, up to just below the new waterline, I guess that weighs nothing also, right? After doing this several more times, the boat is riding just barely above the surface, but all the water in the boat weighs nothing according to you. I guess it is just that last cup of water that fills the boat entirely that "actually" sinks it! That is exactly like "the straw that broke the camel's back". Your argument does not hold water!

You also said, "As a boat goes down at the stern through loss of buoyancy (taking on water), the bow does not rise until the stern actually goes completely under."

Oh, really? Picture this. There are two canoes (a type of boat). In one, there are two 200 lb. men. The canoe is balanced and the waterline is the same distance below the top at both ends. In the other canoe a 50lb. boy is in the bow and a 350 lb. man is in the stern. According to you, the bow will not rise (relative to the position of the waterline of the other canoe) because the stern is not yet "completely under". What are you thinking! This is totally analoguous to the water draining into the stern. Of course the bow will ride higher! For the same overall weight, the heavy stern will ride much lower in the water and the bow will be higher. Or put another way, if the first canoe is balanced and someone hands the man in the rear a 50 lb. cooler which he holds on his lap for the moment, the stern will go down and bow will go up, just like a "see-saw" or "teeter-totter" playground equipment. He better put that cooler in the center, because the water is so near coming over the top (of the stern only), that when he gets handed a beer the canoe will be swamped! Oh, yeah, it was that beer that sank the canoe. The off-center load had nothing to do with it. Yeah, right!

You should have quit while you were ahead. You were correct on your first point. The water does weigh something. You were incorrect on your second point for the same reason. Take your example of the canoe with 2 200lb men. A boat taking on water in the strern would be like a 200lb man in the bow while the man in the back suddenly gained 150lbs. It is not a weight shift, it is an unballanced weight gain. The man in the bow does not suddenly get 150lb lighter and 40 years younger. Yes I realize the irony that a man does not often gain 150lb suddenly either. Maybe we can assume that in the first case his gut was resting on the dock.

There is no hole in the boat at all. The water comes from "sweat". The colder water cools the boat below the water line and the moisture in the air condenses inside the boat and accumulates in the bilge. The amount of water in the bilge never exceeds the waterline due to sweating.

I wish I had a boat.

Wanna buy one?

yeah, but I havn't the allowance for one at this point in my life. Also I would need lots of training as I am after a vessel to navigate the Great Lakes and on out into the North Atlantic starting at Superior or the like.

I've lived on Lake Superior all my life, its a great area. I wish you luck. I'd love to make that trip myself. My grandfather was a bootlegger and scrap hound. He would load up his 1920's 14ft rowboat with iron till he had only 6 inches of freeboard and row it accross St. Louis Bay from Duluth to Superior in 4ft chop. But its probably better to get training and a better vessle.

Like I said, I don't have any experience in open waters, but I would bet that the loaded trip over was better than the empty trip back! I think I would rather have a dence load to get a good draft and put the CG below water line. I assume a 14' wooden rowboat would get tossed around quite easily in 4ft chop, unless she was displacing several hundred pounds.

Juba:

You'd probably enjoy River Horse, by William Least Heat Moon. Tells of his journey across america, by boat, following the path of Lewis and Clark. If you have lots of time, you could build a boat for your purposes. A small but sturdy motorsailer would work well for your trip.

Ken,

Have you read "Blue Highways", his first book? I was living in Columbia, Missouri (where he had been teaching at Columbia College) and bought a signed copy in the MU bookstore.

I read the book in paperback first. He mentions his ex-wife many times throughout the book. I was also the University Choir at Mizzou and went on tour to Great Britain with the choir. Planning to visit my Scottish cousins, I bought the signed copy of "Blue Highways" as a gift for them. Our choir director brought his wife on the trip, an MU English professor. I mentioned my plans to visit Scotland after the tour, and the book. She laughed and said, yes, my ex-husband wrote an excellent book!

STL:

Funny you should mention it. I was just reading a couple of online reviews of "Blue Highways", which I plan to read. (Interesting how these trains of thought meander: Juba's mentioning his desire to travel the great lakes made me remember having read River Horse... which, in turn, made me remember wanting to read his earlier book, the name of which I'd forgotten. So I looked online for a review of River Horse, which mentioned the earlier book... and then a minute or two later, there is your post.)

Great story re the choir director's wife!

One of my favorite quotes is from "Blue Highways". While passing through the Deep South, the author struck up a conversation with a local, an old lady, in a "greasy spoon" restaurant. She asked if he was a Baptist, or if he went to Church, or something like that. He hemmed and hawed a little, start to explain his own personal beliefs, and she interrupted him. "That's all right honey", she said (or something like that). "Everybody down here is Baptist, even the Presbyterians."

Looking at the suggestions so far we have;

1/. Leak being lifted above water line

2/. Leaky check valve on bilge pump

3/. Internal water level equaling outside water level

4/. Osmosis

5/. Leaky prop seal

6/. Condensation

7/. Pressure differential problem

So here is what I think

1/. Not very likely with about a quart or liter of water as it only has a mass of 1 Kg and when compared to the boat this is nothing.

2/. Possible but you would expect the non return valve in the bilge pump/drain to open further as the water level increased.

3/. Not likely again with the small amount of water concerned with.

4/. No as there isn't any water on the other side of the transverse stiffener. If it were water would leak in over the entire hull.

5/. Possible

6/. No for the same reason as 4/.

7/. This could be the answer. The leak is in some way pressure sensitive. Here is one scenario. A small area of fiberglass has delaminated and while there is no water inside the boat there is enough deficiency in the structure to allow water to leak in. When the water level rises slightly inside the pressure, small as it is, is enough to pushes the laminations together again and stops any moor water from leaking in.

I do however have a sneaky suspicion that we are all missing the point.

Masu,

How can you say that "leak being lifted above water line" is not the answer? You base this on your assumption that "mass of 1 Kg and when compared to the boat this is nothing."

What if I told you that the leak is at the bow and almost exactly AT the waterline, but ever so slightly under, so that water WILL leak in. Since all the water that leaks in finds its way to the stern, don't you agree there will be an ever so slight change in the balance of the boat? I suggest that this, ever so slight, change is just enough to raise the bow, ever so slightly, so that the leak raises, just the tiniest amount, above the waterline, and stops leaking water in. When the very small amount of water is removed from the stern, it is lighter and more buoyant and rises, ever so slightly, changing the angle of the waterline a minute amount so the bow dips, just the teeniest bit, and the leak is underwater again. Is that possible? Think about it.

Yes what you say is possible but the 60 Kg person that has moved around would cause the amount of water to change. Remember the person watched one time but not others and depending on where the person moved to could increase the amount of water needed to alter the trim appropriately by as much as 120 Kg.

Ok, I have a boat, the boat has a bilge. The bilge has water. mmmmmm

I vote for this as the most porbable solution.

Most boats are whale shaped. The lowest point under the waterline while sitting still is probably near the windshield (under the ski stowage). Whenever you went to the stern to look at the bilge, some of the water drained from the V shaped bow (and over the transverse ridges) to the fairly flat stern.

As a professional boat builder and a licensed professional chemical engineer, I would have to say that you guys are getting pretty worked up over this one!

The challenge didn't say, but it sure sounds like a seal leak on the outboard cooling loop. If in fact the motor is powered by an inboard engine / outboard drive train configuration, then there is a seal that often fails with time. This seal will cause a leak until the pressure on both sides of the seal is equal. This situation often results in a small amount of water present in the bilge. A very similar situation can arise from a shaft seal leak; whereby water will leak past the shaft until the inside pressure (from water column) is high enough to stop the leak.

Ultimately, given all of the possible answers, the only logical solution is that derived from solving the buoyancy and trim equations with a simplified Bernoulli equation for the balance of water pressure creating the flow. There is some flow path, which by a pressure differential a flow is induced. Once the delta P is eliminated, no more flow.

Stephan Bradley

www.bradleyboatworks.com

There's that darn Bernoulli and boats again. I thought he was the airplane wing guy!

Can you qualitatively describe what is happening with buoyancy and trim when the water has filled the bilge and after it is removed (which starts filling again)?

Thanks Stephan!

Well,

My opinion on the solution was related to good old Bernoulli, but it is Archimedes' Principle that will answer the forces question. His equations define the buoyancy as the force exerted in an upward direction equal to the weight of water displaced. Since the density of sea water and fresh water differ slightly, this force is dependant on the location of the boat, but I digress.

If we assume the boat is a perfect box, then the location of the upward buoyancy force (CB) and the downward weight force (CG) of the boat are equal and located in the center of a box in plan view. This box shaped boat would be in perfect trim. If we move the center of gravity of the box in some way (a person walks out to the edge of our box) then the center of gravity changes and the boat will adjust trim until these forces are in equilibrium.

In the real world of boats, the shape of the boat usually changes from front to rear (fore and aft for you boaters) quite significantly and not uniformly. Trim is determined by balancing the two forces in a real boat.

This is actually a complicated problem that involves an integration of the surface area of the hull to generate the change in displacement volume as you move from the center of mass. This discussion is an entire course section in the school of yacht design, so I can't possibly explain it all, but the result is the same as the above diagram. As the CG moves around in the boat due to changes in weight (such as a person or some water) the boat will adjust trim until the two forces are resolved by the following equation: CB = A x CG (where A is the distance between the two forces).

Someone else mentioned this in an example of people in a canoe. The diagram above demonstrates his argument. When all the forces balance properly, you get the final image. In the first one, the weight is too far forward. In the middle one, the weight is too far aft.

Now, to answer the question presented me, the water that fills the bilge would in fact adjust the boat trim by bringing the center of mass of the boat toward the aft of said boat. Is it enough to move a hole from below the water line to above the water line? I don't know the answer to that because it depends on the total mass of the boat, the mass of water added to the bilge, and the distance from the center of buoyancy. My guess would be no, but it could easily work in a small boat. As someone else pointed out, we are likely assuming a rather large power boat with respect to the small amount of water removed. I doubt the trim would be much for that small amount of water, especially with the person standing over the location of that small weight in part of the original problem.

Man, I hope you guys could stay with me through all of that!!

I agree with the 'sweat theory'. If it were a leak, there are many more symptoms and possibilities you may have noticed earlier. If the boat is docked in salt water, try tasting a bit at the tip of your tongue.

The problem with the condensation theory, is that it would not stop at a certain level in the bilge (unless the bilge pump keeps it at a certain level - not likely).

Condensation would occur in proportion to draught and depth below the floorplate. Below the floorplate is the warm area and the floorplate would be source of temp. differential. Saturation is reached at the balance of vapour pressures. I would think, it is in proportion to enclosed area beneath the floorplate or enclosed volume below floorplate.

Sources of warmth such as motor, sunlight, fenestration, lighting etc are additional factors.

This is a common occurance in forepeak lockers, firepump rooms, chain lockers on ships.

However, condensation would not explain why the water quickly reappeared and then quickly stopped flowing in. Nor would it explain why he said it seems to be a leak, rather than drips slowly running down into the pool of water. I have to agree with poster #4's icecube tray theory (and my own post saying essentially the same.)

Granted, the absence of limber holes connecting compartments is unusual. (In this case, if there were such limber holes and a leak below the water line, the boat would sink if it didn't have other measures to prevent that.) Stiffeners (floors, as they are called) that reach all the way up beyond the waterline are also not common. But I suspect that the poster of the challenge was seeking a simple answer demonstrating a simple physics concept, and may have been a little loose with the actual details of typical boat construction. Drill a hole in the bottom of an ice cube tray, and it will float just fine. The effect is used in boats to enable through hulls, such as sounders and knotmeters to be removed while the boat is in the water: the device is mounted in a well bonded to the hull, with the top of the well above waterline.

I happen to have a 21 foot Luger Southwind Sloop that suffers from this very problem. I normally trailer the boat to and from the lake and the bilge is dry when launched. After about 2 hours it has accumulated almost 2 inches of water in the bilges. The previous owner had the boat tied up to a buoy all last summer and the only water intrusion problem he had was from rain as the hatches do not seal well.

I suspect your problem is different from the one stated in the challenge, but your solution may be one presented as a solution to this challenge, i.e, vapor condensation, rubber boot or valve leakage, etc.

Static conditions, being anchored or moored to a buoy are very different that conditons under sail. You may have a leak that only manifests itself under pressure or vibration caused by the dynamics of sailing.

You may also have a situation where a seal or some type of packing is dry before launch, and allows leakage, but swells up and stops leaking only after several hours in the water. This is sometimes typical of wooden hulls especially with lapstrake construction. Thus, dry bilge before being put in, leaking for a while, but leak stops after some time in the water, allowing the boat to ride at rest in the water without sinking.

It may be nothing, but better get it checked out before it becomes a worse problem, e.g. small crack propogating to become a failed seal.

Hurdent:

I'd bet it leaks at the pivot bolt for the swing keel. Usually this bolt is above the static waterline, so it wont leak in calm water. But once you are sailing, water sloshes up around the bolt, and leaks in. It's a guess, but worth checking.

Well, blow me down! (Sailor talk!) I think I better take another tack (More sailor talk!)

After re-reading the original challenge, I realized that this may not be a continual, never ending process, which is what I assumed in my earlier response. In fact, water was removed from the bilge exactly twice only!

Guest #4, Philp, Blink, and Evar were all partially right, and all partially wrong. The solution has nothing to do with condensation, waterlines, trim, etc. but everything to do with flow from one level to another, the levels separated by the transverse members, but joined by drainage holes through the members!

In a good boat design you want all water that might come into the boat, whether through pin-hole leaks, "shipping" of water over the side from wave action or leaning, rain storms, etc. to drain to one point, the bilge, so it can be easily collected and pumped back out.

Now, since "the interior of the bilge slopes upward from this point and has several transverse ridges in it", these ridges serve as small dams and each maintains a reservoir, but for only a short period of time, as the ridges also have drainage holes to allow the water to flow to the lowest point.

When the lowest point of the bilge is full of water, in equilibrium, the adjoining reservoirs also contain water at the same height. Removing that water from the low spot changes the equilibrium and water will flow into the low spot until a new equilibrium is reached, and you "notice that the water is back". HE NEVER SAID IT HAD REACHED THE SAME LEVEL AS BEFORE!

So when the water is sponged out the second time (should not have been as much, but he doesn't say) and you "watch with horror as more water flows into the low spot in the bilge", it is really the successive reservoirs reaching a new and lower common level, where "the water level doesn't increase beyond a fixed point". This is the same effect as baffles that are put into water beds and other tanks to allow flow, but to keep the water from just sloshing around very quickly. So the transverse "ribs" act as baffles.

The water stops flowing and the level doesn't increase because it has reach a new equilibrium after refilling the bilge. I believe if the sponging was repeated a few more times that eventually you would get a very dry bilge that would stay dry, unless there was a slow leak somewhere in the boat.

However, this solution works even if there is no leak, because we only removed a small amount of the water each time and only did it twice before stopping!

Wow! valid argument, but I still get the feeling we are missing the point. I usually do with these challenges!!

Maybe we should stick to land sailing, right Ken?

Stephan

Stephan:

Speaking of which, have you made progress on your landsailer? Reading the MSN group has got me fired up to sail on something harder than the usual liquid. But time... so little time.

Not one little tiny bit!! I have been trying to finish a canoe for someone and I bought another darn International Scout (a minor addiction) that is taking some time. Now if I was not having to come into work all the darn time!!!!

Hmmm... We are getting down to the wire on this. The answer will be posted in tomorrow's newsletter. One of us is going to get that $64,000 check. It might be STL... but it might not be.

I think this sentence from the challenge is a key: "You wait a while, but the water level doesn't increase beyond a fixed point." I think the fixed point has to be the waterline: the same fixed point to which the water rose prior to the first sponging. I think the limber holes were left out or are completely clogged, and we have a simple leak into one compartment whose walls extend above the waterline. Also supporting this contention is the last sentence, which is, after all, the real question: "Why doesn't the boat fill with water and sink?" I ask you, ladies and gentlemen of the jury: Are we to answer that there is no leak?? Would that not be a let down?? Would that not rob the story of its implied peril, its excitement, its raison d'etre? No, of course not; we can't answer that way! I believe there was leak, a real leak, the kind of leak that could send a boat straight to the darkest depths. The kind of leak that can make grown men turn to jelly. But… a leak stemmed by our hero, separate compartments with partitions extending above the waterline!

I agree, but REMEMBER THE TITANIC!!!!

buoyancy and center of gravity is the simplest answer you move the weight around then you change the center of gravity and water line you can even do it with a dry bilge.

Just sell the boat while it is on the trailer....then buy a different boat after trailering to lake and test driving.........simple as pie....next!!!

In most boats the bilge is designed to hold a small amount of water whenever the boat is afloat. This is designed to prime the bilge pump when one is present. It will not exceed the designed level because it seeps in through a pressure sensitive valve that closes when the level reaches a certain point.

Yo HO Jim Lads,

I like to think that there boat in question has a large hold full of Rum. That be the case, I also like to think that this ship is mine, along with all the Rum! I would invite ye all to sit in me bilges and discuss how the #$%^ that water keeps entering my ship while we drink my Rum. Savvy?

This one seems played out, so here's a micro challenge:

If a dock is the water in which a floatable vessel floats (which it is), then how was Otis able to be "sittin' on the dock of the [SF or any] bay, wastin' time"? Was it just his grandiose delusion? Or perhaps a Messiah complex? Other?

How do you figure that "a dock is the water in which a floatable vessel floats" or if that is valid, that it is the only valid definition? Merriam-Webster has this:

Main Entry: ⁴dock
Function: noun
Etymology: Middle English dokke, probably from Middle Dutch docke
1 : a usually artificial basin or enclosure for the reception of ships that is equipped with means for controlling the water height
2 : ²SLIP 1b
3 a : a place (as a wharf or platform) for the loading or unloading of materials b : a usually wooden pier used as a landing place or moorage for boats

I think Otis was talking about definition #3, sitting on a pier or wharf, "watching the tide roll away", not hovering over a body of water, as you imply. Messiah complex, indeed! I would think that sitting on the dock, resting your bones, watching the ships come in, then watching them go away again, would be wasting time, unless you could turn that experience into a hit song!

You have a leak in your boat in the section of hull between the transverse stiffeners. The boat is a reasonable size with considerable bouyancy. The water flows in until it equals the external water line but there is not enough mass of water to push the boat deeper into the water and cause the positive feedback that would sink the boat.

One assumes the bracing is fairly close together relative to the size of the boat.

Known facts about this problem:

"The interior of the bilge slopes upward from this point and has several transverse ridges in it that serve to stiffen the fiberglass hull. You sponge out about a quart of water."

"You sponge out the water again and watch with horror as more water flows into the low spot in the bilge, seemingly from a hole in the hull."

OK, seems we have established there is a hole in the hull actually visible, and it's easily letting about a quart of water aboard into an area segregated by transverse ridges. Well, if I'm aboard a cruise ship I'd expect there is a swimming pool on the top deck but I don't expect it is going to sink an oceanliner. The physics answer would be about displacement. The boat has sufficient displacement to allow far more weight aboard than a quart of water.

You are the only person aboard this boat at the time. Load this same boat heavily, and that increases pressure on the hull against the water which increases the amount of water that comes aboard. Load it enough so it allows an amount of water that transcends the height of many of those ribs and the real trouble starts. Keep this boat loaded heavily in that manner, and the amount of water allowed aboard combined with the weight of vessel and cargo can surpass displacement, causing it to sink.

The hole in the hull allows water to enter until it reaches the same level as the waterline, but the segregation stops it from spanning the entire length of the vessel. An unimpeded influx of water spanning the length of the vessel changes the displacement characteristics as it fills.

The same theory was used for placing watertight bulkheads in Titanic that only rose partially up the height of the hull. If Titanic had only sustained damage to a couple bulheads, she had sufficient displacement to survive the collision. The sidelong collision allowed water into many compartments, changing the displacement so drastically, she tore herself in two from the stress of her own unsupported weight when the stern rosde out of the water.

Oh, and if the volume of water removed by the bilge pump(s) can easily outpace the volume allowed through the hole under the worst cargo weight pressure situation, then your only worry is that the pump continues without failure until the boat comes out of the water for repair.

Nice analysis, Jim, but based on an invalid assumption.

You did quote the challenge accurately, "You sponge out the water again and watch with horror as more water flows into the low spot in the bilge, seemingly from a hole in the hull." However, you must have missed something, the word "seemingly", when you said,

"OK, seems we have established there is a hole in the hull actually visible..."

Maybe it "seems" to you there is a hole in the hull actually visible, but that is NOT what the challenge said. It said that the water flowed into the low spot, seemingly (not ACTUALLY) from a hole in the hull."

Merriam-Webster says the word seeming (from which comes seemingly) means:

Main Entry: ²seeming
Function: adjective
: outwardly or superficially evident but not true or real <the seeming immortality of our heroes>
synonym see APPARENT
- seem·ing·ly adverb

Hmmmm...."outwardly or superficially evident but not true or real..."

Yes, most of us missed that too. Those answers that argue the "hole" point, missed the "whole" point! My first answer was based on the same assumption that yours was, that the hole was real. Please see my posting (#32) above for my revised answer, that DOES fit the statement in the challenge!

Re answer 32:

You're quite correct, boats should have small holes along the keelson to allow drainage between sections but some don't. The only problem with this answer is that it assumes that the adjoining sections of the hull are not visible but in all fairness it does say that "you pop the hatch" so it could be that you can't see the pockets of water either side of where you are sponging. In which case the water comes from condensation and drains down to the lowest point as per your answer. Personally if I was sponging water out of one section I'd be looking at the others but it doesn't state that.

The level is set by either the bilge pump float or proximity control, and the water stopped appearing because it stopped raining